Microlens

ABSTRACT

An optical fiber assembly has a microlens joined to the face of an optical fiber. The microlens is made from a material which, when liquid, adheres to the face. The microlens can have a focal point which defines an optical path between the surface of the microlens and the optical fiber&#39;s core. This assembly can be formed by applying liquid such a pre-dispensed droplets of liquid to the optical fiber so that the liquid adheres to the optical fiber as a droplet at the face, and solidifying the droplet to form the microlens. The droplet&#39;s shape can be altered as it solidifies.

FIELD OF THE INVENTION

[0001] The present invention is directed generally to the constructionof optical fibers, and, more particularly, to the formation of anoptical fiber having a terminal lens.

BACKGROUND OF THE INVENTION

[0002] Photonic devices often employ optical fibers to guide efficientlyand control light passing therebetween or therethrough. Morespecifically, the optical fibers can transfer light between opticaldevices, guide light to components in the device, transfer light toother optical fibers, or receive light from components in the device orother optical fibers. Such optical fibers typically have alight-transmitting core surrounded by a light confining cladding. Thecore and cladding have diameters on the order of 8-150 μm and 100-700μm, respectively, depending on the type of the fiber (single or multimode) and fiber material (glass or plastic).

[0003] Although light can enter or exit the core of an optical fiberdirectly, the small size of the core means that precise alignment of thecore and the light's source or destination will be required. One way tocomply with optical fibers' precise alignment requirements is to place acollimating lens near the end of the optical fiber; the lens has opticalproperties and is positioned such that light which would otherwise notenter the optical fiber core is directed into the center of the opticalfiber. That is, the lens guides light into the optical fiber's core.

[0004] One known mounting scheme affixes a microlens to the end of theoptical fiber. While this arrangement can comply with optical fiber'sstringent alignment requirements, the procedure for mounting themicrolens on the optical fiber itself complicates the manufacturingprocess; if not done properly, the optical fiber and lens will not becoupled correctly, reducing optical performance. Since the effectivecoupling of fibers and lenses is required in a wide range of photonicapplications, such as detectors/lasers, cross-connect devices, etc.,great care will have to be taken when using this technique to join thefiber and lens to insure proper alignment and mounting.

[0005] At the present time, coupling of the optical fiber and lens isperformed manually. Owing to the close tolerances and precise alignmentsinvolved, this presents substantial challenges. Manually mounting themicrolens to the optical fiber generally is a slow and expensiveprocedure, in part because it is done using active alignment of theoptical components, and in part because only one microlens and fiber canbe joined at a time. Further, constant quality control supervision andchecking of every microlens/fiber pair may be required to insure thatthe resulting products, which are individually fabricated, are ofuniform quality and all possess the required optical characteristics.

[0006] While it is known to form lenses on optical fibers by dipping theoptical fibers into liquid, it is difficult to control precisely theamount of the liquid that is applied to the fiber. Consequently, it isdifficult to form consistent-size, precisely dimensioned lenses onoptical fibers simply by dipping the fiber ends into liquid.

[0007] Thus, there exists a need for a fast, precise and inexpensivesystem for affixing microlenses to optical fibers.

SUMMARY OF THE INVENTION

[0008] The present invention is directed to the arrangement andfabrication of an optical fiber assembly having an optical fiber and amicrolens joined to the face of the optical fiber, the microlens beingmade from a pre-dispensed droplet of liquid, which liquid maintains itsdroplet shape and adheres to the face. The microlens can be shaped toguide light between its surface and the core of the optical fiber.

[0009] The optical fiber assembly can be made by applying apre-dispensed droplet of liquid to the optical fiber, the liquid havingproperties such that the droplet is stable and holds its shape untilcontacted by the optical fiber. The liquid is adhered to the opticalfiber as a droplet at the optical fiber's face, and the dropletsolidified to form the microlens on the face of the optical fiber. Ifdesired, the shape of the droplet can be changed as it solidifies, forexample, by an applied electrical field. Changing the droplet's shapechanges the resulting microlens' optical properties.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In the drawing figures, which are not to scale, and which aremerely illustrative, and wherein like reference characters denotesimilar elements throughout the several views:

[0011]FIG. 1 is a front cross-sectional view of a microlens assemblyformed in accordance with the present invention;

[0012]FIG. 2 is a front cross-sectional view of a second embodiment ofmicrolens assembly in accordance with the present invention, wherein anoptical fiber is bordered by a ferrule;

[0013]FIG. 3 is a front cross-sectional view showing the passage oflight through the microlens assembly of FIG. 2;

[0014] FIGS. 4A-D are views of wire-mesh models showing how variousparameters affect microlens formation; and

[0015]FIG. 5 is a front elevational view showing a number of opticalfibers being dipped into liquid to form microlenses thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0016]FIG. 1 depicts a microlens assembly 1 prepared in accordance withthe present invention. As will be explained in greater detail below, thepresent invention involves both the structure and fabrication of a newtype of microlens assembly 1.

[0017] This microlens assembly 1 includes an optical fiber 2, in whichcladding 3 surrounds core 5. By way of non-limiting example, the core 5could be on the order of 9 μm in diameter, and the cladding 3 could beon the order of 125 μm in diameter for a typical glass single-modefiber. Also by way of non-limiting example, for a typical plasticmulti-mode fiber (such as Lucent's Lucida® prototype plastic fiber) thecore 5 could be on the order of 120 μm in diameter, and the cladding 3could be on the order of 200 μm in diameter. The core 5 and cladding 3terminate at face 9, which is preferably both flat and orientedperpendicular to the axis of the fiber. Flat face 9 can be prepared inknown fashion. Optical fibers of the type just described are themselvesknown and commercially available, and this invention is also applicableto any suitable fibers which are now known or hereafter developed. Sinceby itself optical fiber 2 is conventional, the precise opticalproperties of the cladding 3 and core 5 which enable the transmission oflight through optical fiber 2 are themselves known, and so need not bediscussed in detail herein.

[0018] With continued reference to FIG. 1, microlens 7, which has asurface 4, is joined to optical fiber 2 at face 9. Microlens 7 has afocal point FP which is preferably located on or near face 9 at thecenter of core 5 (while it is presently thought to be preferable to havethe focal point located right at the edge of the fiber, depending uponthe optimal launch conditions for a particular fiber, the focal pointalso could be located some distance away from the fiber edge. By way ofnon-limiting example, the microlens 7 could be approximately 2.7 mm longby approximately 1.4 mm in diameter at its widest point and the surface4 of the microlens 7 could have a curvature of approximately 0.8 mm.

[0019]FIG. 2 depicts an alternate embodiment of the present inventionwherein microlens assembly 101 includes an optical fiber 102 having acore 105 surrounded by cladding 103. Cladding 103 is in turn surroundedby ferrule 111 which serves to support and strengthen the optical fiber102. By way of non-limiting example, ferrule 111 can have a diameter ofapproximately 1.25 mm, and be approximately 6 mm long. Typicallyferrules are made out of ceramics, glass, metal, or plastic. Microlens107 is attached to optical fiber 102 at face 109. As shown in FIG. 2,the microlens 107 has a focal point FP located on face 109, preferablyat the center of core 105 of the optical fiber 102.

[0020] Turning now to FIG. 3, a beam of light 113 is shown striking thesurface 104 of microlens 107 and traveling into the microlens 107. Beam113 is preferably coherent and the light rays of the beam 113 arepreferably parallel. Owing to the curvature and composition of microlens107, the light refracts at surface 104 in accordance with knownprinciples of optics and is thereby redirected toward the focal pointFP. Since focal point FP is located at the center of the core 105, thelight enters core 105 and propagates along the optical fiber 102 inknown fashion. The light propagating through the optical fiber 102 canbe coherent and the light waves essentially parallel.

[0021] It also will be understood that light can travel through thisinvention in the reverse manner. In the example depicted in FIG. 3,coherent light passing through the optical fiber 102 travels along core105, leaves the core 105 at focal point FP and enters microlens 107. Thelight then travels through microlens 107 to the surface 104 of microlens107, where, owing to the optical properties of the microlens 107 thelight refracts and leaves as beam 113. Microlens 107 can be suitablydimensioned so that beam 113 emerging from surface 104 is generallycoherent and the light waves are parallel.

[0022] It will be appreciated that light travels through the embodimentof this invention shown in FIG. 1 in the same manner as has beendescribed in connection with FIG. 3.

[0023] Next, schemes for forming microlenses on optical fibers inaccordance with the present invention will be described.

[0024] By way of non-limiting example, and with reference to FIG. 1,microlens 7 can be formed from a pre-dispensed droplet of liquid 21 suchas the melt of a polymer or a monomeric liquid. Such material should besufficiently stable for the dispensed droplet 21 to hold its shape afterformation, until contacted to the optical fiber 2. Examples of suchmaterials include poly(methyl methacrylate) (PMMA) and other transparentacrylic polymers. The pre-dispensed droplet of liquid 21 is preferablyapplied to the face 9 of the optical fiber 2 by positioning the opticalfiber 2 in a generally vertical orientation with the face 9 of theoptical fiber 2 pointing generally downward. The optical fiber 2 is thenlowered in the direction of arrow A toward a non-sticking surface of acontainer 17 containing the pre-dispensed droplet of liquid 21 so thatthe end face 9 of the optical fiber 2 contacts the pre-dispensed dropletof liquid 21. It is presently thought to be preferable to move theoptical fiber 2 directly downward until the end face 9 of the opticalfiber 2 contacts the surface of the pre-dispensed droplet of the liquid21. The optical fiber is then raised upward so that a droplet 15 of theliquid 21 having the desired size and shape adheres to the end face 9 ofthe optical fiber 2.

[0025] As shown in FIG. 1, container 17 has a raised edge 17′ whichhelps to confine pre-dispensed droplet 21. Other arrangements could beused; for example, a concave or “bowl-shaped” container 17 also could beused. Likewise, different height edges 17′ could be employed. Anysuitable non-stick surface 18 which allows the pre-dispensed droplets tomaintain their shape without wetting the inside of the container 17could be used.

[0026] After a droplet 15 having the desired shape is formed, thedroplet 15 is solidified. By way of non-limiting example, this can bedone through cooling in the case where the droplet 15 is made frompolymer melt, or by a polymerization reaction in the case where thedroplet 15 is made from a monomeric liquid. Any other suitable techniquefor hardening the droplet 15 also could be used.

[0027] The shape of the droplet 15 which becomes the lens 7 isdetermined by the interplay of such factors as the volume of the liquiddroplet 15. As shown in FIGS. 4A-D, by appropriately selecting thevolume of the droplet 15 it is possible to adjust the shape of thedroplet surface 4 which will act as microlens 7 (FIG. 4D establishes thecoordinate X-Y axes which are used)..

[0028] The volume of the droplet 15 can be selected based upon thefollowing considerations: the diameter of the optical fiber 2 or, if asshown in FIG. 2 a ferrule 111 is used, the diameter of both the opticalfiber 102 and the ferrule 111, the refractive index of the liquidforming the droplet 115; the specific density of the liquid forming thedroplet 115; the surface tension of the liquid forming the droplet 115;in the case where a polymer melt is used, the coefficient of thermalexpansion of the molten liquid and the temperature dependence of itsrefractive index or, in the case where the droplet is formed from amonomeric liquid, the polymerization shrinkage of that monomeric liquidand its refractive index change due to polymerization; the surfacetension of the liquid which becomes the microlens 2 or 102; and, theforce of gravity.

[0029] If desired, the shape of the droplet 15 also can be altered byusing electrostatic force to deform the droplet 15 before or during theprocess of its hardening into the microlens 7. The electrostatic forcecan be generated by charging a pre-dispensed droplet of liquid 21. Theapplied electric field E then exerts electrostatic force on the droplet15 which is proportional to its charge, and that electrostatic forcewill alter the shape of the liquid droplet 15 as it hardens into thelens 107.

[0030] Alternatively, one can apply electric field E without chargingthe droplet. In this case the droplet elongation will be proportional tothe dielectric susceptibility (and thus to dielectric permittivity) ofthe droplet material.

[0031] It will be appreciated that the droplet 15 can be elongated bysuitably changing the magnitude and direction of the applied electricalfield E, and the extent to which the droplet 15 is deformed can becontrolled by suitably selecting the magnitude of the applied electricalfield E. More specifically, where an applied electrical field E is usedto deform the droplet 115, the absolute value and direction of theelectric field vector, droplet charge, and the dielectric permittivityof the droplet material will affect the force applied to the droplet115. The exact value of the force deforming the droplet 115 can beeither calculated using standard equations of electrodynamics, or, inmany practical settings, determined experimentally for a given dropletsize, material, and desired elongation.

[0032] Although the applied electrical field shown in FIG. 1 is depictedas being vertically-oriented and is thought to be preferable, otherfield orientations are contemplated and within the scope of thisinvention.

[0033] With reference to FIG. 2, microlens 107 can be formed on the endface 109 of optical fiber 102 in the manner just described.

[0034] With reference now to FIGS. 4A-D, and for the purposes of thisinvention, the shape of a liquid droplet 215, 315, 415, can be modeledusing the following equations to perform a quantitative analysis of thedroplet shape: $\begin{matrix}{{\frac{d^{2}\hat{x}}{d\quad y^{2}} - {\left( {1 - {\gamma \quad \hat{y}}} \right)\left\lbrack {1 + \left( \frac{\hat{x}}{\hat{y}} \right)^{2}} \right\rbrack}^{3/2} - {\frac{1}{\hat{x}}\left\lbrack {1 + \left( \frac{\hat{x}}{\hat{y}} \right)^{2}} \right\rbrack}} = 0} & (1)\end{matrix}$

[0035] where

{circumflex over (x)}=βx  (2)

ŷ=βy  (3) $\begin{matrix}{\frac{1}{\beta} = \sqrt{\frac{\gamma}{\alpha}}} & (4) \\{\alpha = \frac{\rho \quad g}{\Gamma}} & (5)\end{matrix}$

[0036] where γ is an arbitrary dimensionless parameter which determinesthe characteristic size of the droplet, ρ is specific gravity of theliquid, Γ is the liquid surface tension, and g is acceleration due togravity, taken with a negative sign.

[0037] In order to obtain the shape of the droplet Eq. (1) should besolved with the following boundary conditions:

{circumflex over (x)}(ŷ=0)=0 $\begin{matrix}{{\hat{x}\left( {\hat{y} = 0} \right)} = \left. {0\quad \text{and}\quad {\frac{\hat{x}}{\hat{y}}}_{\hat{y} = 0}}\rightarrow\infty \right.} & (6)\end{matrix}$

[0038] Focal length of the droplet described by Eq. (1) with boundaryconditions (6) is defined by the following equation: $\begin{matrix}{\hat{f} = {2\left( {1 + \frac{1}{n - 1}} \right)}} & (7)\end{matrix}$

[0039] where n is refractive index of the droplet material.

[0040] The volume of the droplet that has its overall length exactlyequal to its focal length is determined as: $\begin{matrix}{\hat{V} = {\pi {\int_{0}^{\hat{f}}{\left( {\hat{x}\left( \hat{y} \right)} \right)^{2}{\hat{y}}}}}} & (8)\end{matrix}$

[0041] where {circumflex over (V)}=β³ Volume and Volume is the dropletvolume.

[0042] Several examples of the possible solutions of Eqs.(1)-(8) areshown in FIGS. 4A-4C. Each of the droplets 215, 315, 415 depicted inFIGS. 4A-C has a neck region 221, 321, 421 and a bulge region 223, 323,423. The width of the base of the droplets 215, 315, 415 is ŵ, theoverall length is {circumflex over (l)}, and the volume is {circumflexover (V)}. The droplets are shown in dimensionless coordinates{circumflex over (x)} and ŷ. In order to translate them to the actualcoordinates x and y one needs to use Eqs. (2)-(5). For the case wherethe liquid is a PMMA melt (Γ=33·10⁻³ N m⁻¹, ρ=1.18·10³ kg m⁻³, n=1.49)such translation results in the following

[0043] 1. for γ=−0.15 (FIG. 4A) we have 1/β=653 μm and thus:

[0044] {circumflex over (l)}6.08 translates into l=3977 μm

[0045] ŵ=5.21 translates into w=3407 μm

[0046] {circumflex over (V)}=90.17 translates into V=25.2 μl

[0047] 2. for γ=−0.10 (FIG. 4B) we have 1/β=534 μm and thus:

[0048] {circumflex over (l)}=6.08 translates into l=3247 μμm

[0049] ŵ=2.68 translates into w=1431 μm

[0050] {circumflex over (V)}=61.99 translates into V=9.4 μl

[0051] 3. for γ=−0.07 (FIG. 4C) we have 1/β=447 μm and thus:

[0052] {circumflex over (l)}=6.08 translates into l=2717 μm

[0053] ŵ=1.41 translates into w=630 μm

[0054] {circumflex over (V)}=49.36 translates into V=4.4 μl

[0055] Comparing FIGS. 4A-C, it can be seen from the droplets shown thatthe droplets 215, 315, 415 become progressively more contoured; the neck421 of droplet 415 is much more pronounced than the neck 221 of droplet215. The droplet 415 might be suitable for the use with the plasticoptical fiber such as Lucida® fiber described above. On the other hand,the droplet 315, which has a larger neck, might be suitable for the usewith the fiber enclosed in a ferrule, similar to the one, describedabove.

[0056] This invention lends itself to the fabrication in quantity ofmicrolens assemblies. One embodiment for manufacturing multiple opticalfibers with microlenses mounted thereon is depicted in FIG. 5 (forclarity, only portions of the optical fibers are shown). As depictedtherein, a group of optical fibers 502, 502′, 502″ . . . 502 ^(n) areeach secured to a frame 519 such that the faces 509, 509′, 509″ . . .509 ^(n) of the optical fibers 502, 502′, 502″ . . . 502 ^(n) projectdownward beneath the frame 519. The frame 519 is then lowered in thedirection of arrow B so that the faces 509, 509′, 509″ . . . 509 ^(n)are brought into contact with the pre-dispensed droplets of the liquid521 that, when solidified, will form the microlenses (not shown). By wayof non-limiting example, in the embodiment depicted in FIG. 5, the frame519 can be lowered until the faces 509, 509′, 509″ . . . 509 ^(n) of theoptical fibers 502, 502′, 502″ . . . 502 ^(n) are just touch the surfaceof the pre-dispensed droplets of the liquid 521.

[0057] In the same manner as the embodiment depicted in FIG. 1,container 517 shown in FIG. 5 has a raised edge 517′ which helps toconfine pre-dispensed droplets 521. Other arrangements could be used;for example, a concave or “bowl-shaped” container 517 (not shown) alsocould be used. Likewise, different height edges 517′ could be employed.Any suitable surface 518 which allows the pre-dispensed droplets 521 tomaintain their shape without wetting the inside of the container 517,such as a non-stick surface, could be used.

[0058] The present invention offers the following advantages whencompared with existing techniques for attaching microlenses to opticalfibers.

[0059] During formation in accordance with the present invention,surface tension of the liquid applied to the optical fiber will causethe microlens to be automatically aligned with the center of the fibercore, and the focal length of the lens is adjusted to the edge (endface) of the fiber. Thus, the expensive equipment and slow alignmentprocedure of traditional processing required to achieve such positioningcan be avoided.

[0060] A further benefit of the present invention is that the surface ofthe lens material does not contact foreign objects, ensuring that thelens surface will be very smooth. This should reduce scattering lossesof the lens.

[0061] The present invention is inherently parallel, allowingsimultaneous formation of many microlenses on a fiber array or ribbon.

[0062] The present invention may be very cost effective and should notrequire expensive equipment or materials.

[0063] Thus, while there have been shown and described and pointed outfundamental novel features of the invention as applied to exemplaryembodiments thereof, it would be understood that various omissions andsubstitutions and changes in the form and details of the disclosedinvention may be made by those skilled in the art without departing fromthe spirit of the invention. It is the intention, therefore, to belimited only as indicated by the scope of the claim appended hereto.

What is claimed is:
 1. An optical fiber assembly, comprising: an opticalfiber having a face; and a microlens joined to the face of the opticalfiber, the microlens being made from a pre-dispensed droplet of a liquidwhich adheres to the face.
 2. An optical fiber assembly according toclaim 1, wherein the microlens directly contacts the face of the opticalfiber.
 3. An optical fiber assembly according to claim 1, wherein themicrolens has a surface and the optical fiber has a core, and themicrolens has a focal point such as to establish an optical path betweenthe surface of the microlens and the core.
 4. An optical fiber assemblyaccording to claim 1, wherein the microlens comprises a transparentpolymeric material.
 5. An optical fiber assembly according to claim 1,wherein the transparent polymeric material comprises an acrylic polymer.6. An optical fiber assembly according to claim 1, wherein thetransparent polymeric material comprises PMMA.
 7. An optical fiberassembly according to claim 1, further comprising a ferrule surroundingat least a portion of the optical fiber.
 8. A method of preparing anoptical fiber assembly, comprising the steps of: applying apre-dispensed liquid droplet to an optical fiber having a face, thedroplet adhering to the optical fiber at the face; and solidifying thedroplet to form a microlens joined to the face of the optical fiber. 9.A method of preparing an optical fiber assembly as in claim 8, furthercomprising the step of altering a shape of the droplet at least one ofbefore and during the step of solidifying.
 10. A method of preparing anoptical fiber assembly as in claim 9, wherein the step of alteringcomprises the steps of: applying a charge to the pre-dispensed liquiddroplet; and generating an electric field around the droplet, whereinthe electric field interacts with the charge on droplet to apply forceto the droplet, thereby altering the shape of the droplet.
 11. A methodof preparing an optical fiber assembly as in claim 8, wherein the liquidhas at least one property which affects a shape of the droplet duringthe step of solidifying.
 12. A method of preparing an optical fiberassembly as in claim 11, wherein the shape of the droplet is determinedby at least one of a volume of the droplet, a diameter of the opticalfiber, a diameter of the optical fiber, a refractive index of theliquid, a specific density of the liquid, a surface tension of theliquid, and gravity.
 13. A method of preparing an optical fiber as inclaim 8, wherein the liquid is a monomeric liquid; and the shape of thedroplet is affected by a surface tension of the liquid.
 14. A method ofpreparing an optical fiber as in claim 8, wherein the optical fiber isone of a plurality of optical fibers supported by a movable frame.
 15. Amethod of preparing an optical fiber assembly as in claim 9, wherein thestep of altering comprises the step of generating an electric fieldaround the pre-dispensed droplet, wherein the electric field interactswith the droplet to apply force to the droplet, thereby altering theshape of the droplet.
 16. An optical fiber assembly prepared by themethod of claim 8, and having an optical fiber including a face, and amicrolens joined to the face of the optical fiber, the microlens beingmade from a material which, when liquid, adheres to the face.